When a vertebrate—an animal with a backbone—stubs its toe, electrical signals get carried from the toe to the spinal cord by a nerve, which is made up of bundles of long, fiberlike cells.

Since the researchers couldn't study a T. rex's nerves directly, the team looked at how nerves work in a range of modern animals, from the tiny shrew to midsize dogs and pigs to massive Asian elephants.

The scientists found that, for all body sizes, nerves have a basic speed limit of about 180 feet (55 meters) a second. That's the fastest a signal can travel from an animal's feet to its spinal cord—the kind of signal that's essential for walking and running.

At that speed limit, big animals such as elephants can't run too fast or they're effectively running blind.

Suppose an elephant steps on a pebble, said study leader Max Donelan of Simon Fraser University in Burnaby, Canada. If the pachyderm was running fast, "its foot would be nearly off the ground before it could do something in response to that troublesome pebble."

The same goes for T. rex, said study co-author John Hutchinson, an expert on dinosaur movement at the Royal Veterinary College in London.

According to the study, there's a trade-off between the number of nerve cells in a bundle and how fast the nerve can transmit a signal.

For a big animal such as an elephant to be as fast as a shrew and still feel every step, the elephant's nerves would have to be 100 feet (30 meters) thick—clearly impossible.

Instead, elephant nerves can either be relatively slow and sensitive or fast and dulled.

As long as a bus and weighing around 6.5 tons, the average T. rex would also have needed to move slowly to feel with its feet, according to the study, which appears Wednesday in the Proceedings of the Royal Society B.

The idea that T. rex lumbered like an elephant fits with other studies of the dinosaur's body, including one paper that found that T. rex's leg muscles would have to have been heftier than its whole body weight for the dinosaur to have been a speed demon.

"To be agile, Tyrannosaurus would need to be both all muscle and all nerve," Simon Fraser's Donelan said.